1. Field of the Invention
The present invention relates to a variable displacement type compressor. More specifically, it relates to a variable displacement type compressor, of which discharge displacement is varied according to piston stroke by controlling an inclination angle of a swash plate.
2. Description of the Related Art
In general, an engine speed of a vehicular engine is varied according to a load applied to the engine during idling period. When a compressor is mounted on a vehicle, the compressor constitutes a part of the load applied to the engine. A load applied to the compressor influences the engine speed. Therefore, the engine speed should be controlled while the variation of the load applied to the compressor is considered, in order to drive the vehicle comfortably without stalling the engine running during the idling period.
To achieve this type of engine control, Japanese Unexamined Patent Publication No. 62-218670 discloses an apparatus for detecting displacement of a variable displacement type compressor. In this conventional apparatus, a detected piece is attached at the circumference of a swash plate. A housing includes a detector which is disposed between a swinging center of the detected piece and one swinging limit position. The detector detects a period when the detected piece is positioned at right side beyond the detector, and a period when the detected piece is positioned at left side beyond the detector. The ratio of the period (i.e., existing ratio of the detected piece) when the detected piece is positioned at either the right or left sides beyond the detector to a sum of both periods (i.e., a period of time when a swash plate takes to complete a swinging cycle) differs each other according to the displacement of compressors. This allows to calculate the displacement of the compressor.
However, in the conventional apparatus for detecting the displacement, variation of the existing ratio of the detected piece is small with respect to the variation of the inclination angle of the swash plate when the swash plate is in the region of large inclination angle (i.e., the region of displacement is large). Therefore, displacement detection accuracy in the large displacement region is exceptionally unsatisfactory. In other words, output values of detected signals by the apparatus have relatively low linearity. Therefore, even when the detection level of the displacement of the compressor is divided into three stages (i.e., large, intermediate, and small), and the displacement is vaguely detected to categorize into one of three stages, it is rather difficult to detect the displacement.
Therefore, the engine speed of the vehicular engine on which the compressor is mounted can not be accurately controlled during the idling period.
For another type of apparatus which detects the displacement, the apparatus including a plurality of small Hole elements is known. However, this apparatus has drawbacks in which the discrimination and thermal durability are not superior, and the wiring operation becomes troublesome due to the increase of the number of wires for outputting signals.